Portable scales for use in measuring large loads such as heavy vehicles require that they be relatively light weight with a small size and a low profile. The environment in which these portable scales are utilized also requires resonably stable temperature characteristics to provide a reasonably accurate measurement of the weight. Normally, prior art scales have provided some type of zeroing mechanism to compensate for temperature variations. However, this mechanism requires that the scale be allowed to stabilize prior to performing the weight measurement. These types of scales are impractical in a normal environment since the scale may be removed from one temperature medium and suddenly placed in another temperature medium, such as a concrete road, resulting in substantial inherent inaccuracies.
Previously developed scales have utilized either some arrangement of springs or hydraulics to measure vehicle weight. In the past, such apparatus has often been bulky in order to achieve the required degree of accuracy. One device that has provided a relatively small scale is disclosed in U.S. Pat. No. 4,007,800 issued to W. Janach, et al. This scale utilizes a plurality of parallel elliptically shaped hollow spring elements. The spring elements all communicate with a gauge and are filled with a fluid. A plate disposed on top of the spring elements compresses the spring elements when subjected to an external weight. The compression of these spring elements results in a volumetric decrease thereby forcing the fluid therein to be displaced into an external reservoir, which is comprised of an expanding bellows type gauge. The deflection of the bellows is proportional to the deflection of a needle on the gauge. By measuring the volumetric change, the elastic deformation of the spring elements can be converted to a weight measurement.
Although the Janach patent discloses a low profile scale, the use of the elliptical spring elements presents manufacturing and cost problems. This is due to the fact that the elliptical spring elements are essentially tubular members that must be carefully dimensioned and attached to a base on the exterior thereof and then the interiors thereof must be attached to a common channel connecting them to the gauge. In addition, the temperature stability of such previously developed scales has not been completely satisfactory. There thus exists a need for a light weight portable scale with improved accuracy and with temperature compensation, that is relatively simple to manufacture.